It's a military term. The ranks are the lines of soldiers, and if you close them up, you move everyone in together to form a solid mass of soldiers that is hard to penetrate. This figuratively means that everyone banded together to repel an "enemy."
"Dissension in the ranks" is a common (and now overused) idiom.
It means you did not ask them - you kept your mouth closed and swallowed instead of talking.
Went under is used to mean they have failed, as in drowning. A business that went under is one that has closed or gone bankrupt.
It means that they banded together more tightly and supported one another against a common enemy or antagonist.
It's not an idiom because you can figure out what it means. It's just a slangy way of describing sleep. If you don't sleep a wink, you figuratively never closed your eyes, even to wink. It's an exaggeration.
It means things which occur in secret.
It's not really an idiom. It means "what are you thinking about."
RFP is not an idiom. It's an abbreviation.
It is not an idiom, it means your nose is itching.
It's not an idiom. It means the tip of your nostril.
idiom means expression like a page in a book
"Sieve" is not an idiom. See the related link.